1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems use a network of access points such as base stations to provide wireless connectivity to various access terminals, such as mobile units and/or other devices that are enabled for wireless communication. The coverage area of a wireless communication system is typically divided into a number of geographic areas that are conventionally referred to as cells or sectors. The coverage area of each cell in the wireless network is limited by the propagation loss of radio signals transmitted by access points that provide coverage to the cell. Thus, the coverage area of each cell is determined by the location and the transmit power of the access point, as well as the geography of the cell and the location of any interfering objects. For example, the coverage area of a cell may be reduced if a building or a mountain is present near the access point. The boundaries of the cells are not rigidly defined and may vary with time. Thus, coverage areas may overlap such that multiple access points may provide coverage to the overlapping regions, although the strength of the signal provided within the overlapping regions may be different for the different access points.
Access terminals located in the coverage area of a cell may access the wireless communications system by establishing a wireless communication link, often referred to as an air interface, with the access point associated with the cell. Once an access terminal has connected to the system, the access terminal can communicate using unicast (i.e., point-to-point) communication and/or multicast (i.e., point-to-multipoint) communication. One example of a multicast service is the push-to-talk (PTT) service. Subscribers to the PTT service can join groups of access terminals that are able to communicate with each other. When a user of one of the access terminals in the group speaks or transmits other data, the other users within the group can hear the voice transmission or receive the data transmission. Access terminals in a PTT group can be in the same cell or sector or can be distributed across multiple cells and/or sectors.
The resources of the air interface are divided into temporal interlaces. Each interlace is an interval of time that can be allocated to a particular service, access terminal, and/or access point. The interlace structure consists of a time interval corresponding to a predetermined number of interlaces. The interlace structure is then repeated after the previous set of interlaces is over. For example, if the interlace structure includes four interlaces, then the interlace structure consists of the time interval assigned to the first interlace, the time interval assigned to the second interlace, the time interval assigned to the third interlace, the time interval assigned to the fourth interlace, and then the structure repeats again following the fourth interlace.
Each interlace is further subdivided into a plurality of multiplexes. Different communication systems can support different numbers of multiplexes for each interlace. For example, some communication systems can support 4, 8, or 12 multiplexes for each interlace. Typically, one interlace is reserved for control information and the remaining interlaces can be used to transmit voice and/or data. The interlace-multiplex pairs that aren't reserved for control information can be allocated to various services, including broadcast and/or multicast services. For example, a system that supports four interlaces of four multiplexes has a total of 16 interlace-multiplex pairs and 12 of the interlace-multiplex pairs can be used to transmit voice and/or data.
Broadcast and/or multicast services can be transmitted in one or more allocated interlace-multiplex pairs. For example, an access point may be allocated a first interlace-multiplex pair to support a PTT service for a group of users in a sector associated with the access point. The group of PTT users may also include users that are located in other sectors served by different access points. The users located in the different sectors may be able to use the same interlace-multiplex pair but, in some cases, this interlace-multiplex pair may have been previously allocated to a different service. In that case, the users in the other sectors have to be allocated to a different interlace-multiplex pair. Consequently, a single broadcast and/or multiplex call or data flow may need to use several interlace-multiplex pairs to maintain the call or data flow with multiple users in different sectors.
Interlace-multiplex pairs are a scarce air interface resource and the conventional techniques for allocating interlace-multiplex pairs to broadcast and/or multiplex services have a number of drawbacks. For example, the conventional technique for allocating interlace-multiplex pairs can lead to numerous interlace-multiplex pairs in different sectors being allocated to the same call or flow. Allocating additional interlace-multiplex pairs to the same call and/or flow prevents these interlace-multiplex pairs from being utilized to provide other services, which decreases the overall broadcast and/or multicast capacity of the system. Furthermore, using multiple interlace-multiplex pairs to support the same call and/or flow in different sectors can significantly complicate the handoff procedure when an access terminal moves from a sector that uses a first interlace-multiplex pair to a sector that uses a second interlace-multiplex pair. These complications can introduce delays into the handoff process and, in some cases, lead to the user being dropped from the broadcast and/or multicast call or flow.